QUALITATirE INFLUENCE OF CORRELATION 215 



shoots in the vegetative points have their dorsiventrality merely earlier 

 and more firmly induced than usual, and that very young lateral shoots 

 are capable of replacing a terminal one. It may be mentioned here 

 that the spur-shoots of species of pine may in like manner be caused to 

 develop into long shoots, and the same holds for other cases. 



The potato furnishes an example of a specially plastic subject, and 

 Knight carried out a number of very interesting experiments upon the 

 plant. The tubers of the potato are underground lateral shoots which 

 have been transformed into reservoirs of food. Their transformation is 

 brought about under the influence of the material which flows down- 

 wards from the leaves in which it is formed. If the aerial shoots are 

 cut off from a potato-plant before the formation of tubers is begun 

 the subterranean shoots grow into the air and become leafy shoots. 

 They are nothing else then than leaf-shoots which on account of their 

 position in the whole shoot-system of the plant have become accustomed 

 to an underground life, and subsequently under the influence of the 

 material supplied from the aerial leafy shoots have become transformed 

 into tubers. If this double influence be removed they take on at once 

 their original character and replace the aerial organs which have been 

 removed. It is possible also, as Knight has shown, to cause aerial shoots 

 to form tubers. This takes place if at an early period the subterranean 

 stolons are removed, or their connexion with the aerial parts be interfered 

 with. Knight carried his experiments so far that he was able to cause 

 the formation of tubers on the top of the aerial shoots the points which 

 are furthest separate from the normal position of formation of tubers 1 . It 

 is also to be noted that want of light is favourable to the formation of tubers. 



The formation of branch-thorns, in which a shoot-axis by abortion 

 of the leaves and cessation of growth in length becomes a thorn, is quite 

 comparable with the formation of tubers. If one cuts off the end of 

 a branch the side-shoot of which would have developed in normal 

 vegetation into a thorn, this side-shoot will become an ordinary leafy 

 shoot, not a thorn. 



The influence of correlation in the configuration of the leaves is known 

 in a number of cases. The sporophylls of Pteridophyta will be discussed 

 in this connexion in the special part of this book ; but I may here mention 

 that they frequently deviate in form, size, and direction, from the foliage- 

 leaves. These deviations are chiefly occasioned directly by the appearance 

 of the sporangia on the sporophylls -. The primordia of the sporophylls 



1 See also De Candolle, Pflanzenphysiologie,, i. p. 130 ; Vochting, Uber die Bildung der Knollen, 

 in Bibliotheca botanica, Heft 4. Cassel, 1887. 



2 This influence naturally takes effect often very early ; before the sporangia are visible as such 

 the material changes have occurred which lead to their construction and these changes may affect the 

 configuration of the primordium of the leaf. 



